The present invention relates to a process for the coating of a catalyst support, which contains two different partial structures, with a catalytically active coat, and the catalysts obtained by this process.
Catalyst supports which contain two different partial structures are increasingly being used in the area of automotive exhaust gas catalysis. They are chiefly filter bodies for removing soot particles from the exhaust gas, in particular of diesel engines.
In the past, various filter types have been disclosed for this intended use. The so-called wall flow filters are widely used. These are monolithic filter bodies which have a cylindrical shape and through which parallel flow channels for the exhaust gas extend from the entry end face to the exit end face. The filter bodies consist of a uniform porous ceramic material and are produced, for example, by extrusion. In order to impose the filter effect, the flow channels are mutually closed at the entry and exit end faces so that the flow channels are divided into entry and exit channels and, on flowing through the filter body, the exhaust gas has to pass from the entry channels through the porous partitions between the channels into the exit channels. The soot particles contained in the exhaust gas are filtered out of the exhaust gas stream and deposited on the walls of the entry channels.
A further filter type is described, for example, in the documents DE 197 04 147 A1 and DE 201 17 873 U1. For retaining particles from a gas stream flowing through the filter body, the filter bodies of this filter type contain gas-impermeable first layers and gas-permeable second layers, which are arranged so that channels through which the exhaust gas can flow are formed. The gas-permeable second layers consist of porous filter mats, and the gas-impermeable first layers are provided with paddles which project into the channels through which flow can take place and direct the exhaust gas predominantly to the porous second layers and conduct it through these. A particular advantage of these filters is their lower tendency to blockage owing to soot deposits. These filters therefore contain two different partial structures, namely the porous filter mats and the gas-impermeable first layers. Filter bodies of this type are also referred to below as filter catalysts, in order to distinguish them from the wall flow filters.
In addition to the deposition and removal of soot particles from the exhaust gas, a catalytic treatment of the exhaust gas and/or of a component added to the exhaust gas, such as, for example, urea, is frequently desired. For this purpose, the filter can be catalytically coated. These coats may be oxidation-active coats which contain platinum and/or palladium as catalytically active components, or so-called soot ignition coats which contain, for example, cerium oxide and/or vanadium oxide. Coats having other catalytic effects, such as, for example, the storage of oxides of nitrogen and the selective catalytic reduction of oxides of nitrogen with ammonia, are likewise known.
The catalytic coats of these filters are preferably so-called dispersion coats in which the catalytically active components are present in highly disperse form on finely divided support materials, such as, for example, active alumina. This type of coats ensures optimum utilization of the catalytic potential of the catalytically active components. The support materials coated with the catalytically active components are referred to below as catalyst material.
For coating of the filter bodies, the catalyst material is first dispersed in a carrier liquid, generally water. For the coating, for example, the dispersion is then poured over the filter bodies or said filter bodies are immersed in the dispersion or the dispersion is sucked or pumped into the filter bodies. Excess coating dispersion is then removed by blowing out with compressed air or by sucking out. The coat is then dried and/or is calcined at temperatures between 300 and 900° C. In a preferred embodiment, the thermal treatment takes place at between 300 and 700° C. The techniques described here are known to the person skilled in the art in the area of automotive exhaust gas catalysis and therefore require no detailed explanation.
A substantial problem in the coating of filter catalysts is that their two partial structures have a different absorptivity for the coating dispersion. In the case of the filter catalysts, the greatest part of the coating dispersion applied to the filter body is deposited in the porous filter mat. The pores become blocked thereby and thus eliminate the filter effect. The filter body then acts only as a simple flow-through monolith with increased exhaust gas back-pressure.